1. Field of the Invention
This invention relates to an optical information processing apparatus for effecting recording and/or reproduction of information by scanning an optical recording medium with a condensed light beam.
2. Related Background Art
Various forms, such as a disk-like form, a card-like form and a tape-like form are forms of media for recording information thereon by the use of light for and reading out the recorded information therefrom.
For example, in an optical disk, scanning is effected by a light beam modulated in accordance with information to be recorded and is reduced down into a minute spot, whereby information is recorded as optically detectable record pit rows (information tracks). In this case, to record information accurately without causing any difficulty such as intersection between the information tracks, it is necessary to control the applied position of the light spot in the surface of the optical disk in a direction perpendicular to the scanning direction (auto-tracking, hereinafter referred to as "AF"). Also, in order to apply the light spot as a minute spot of a stable size in spite of any bending or mechanical error of the optical disk, it is necessary to effect control in a direction perpendicular to the surface of the optical disk (auto focusing hereinafter referred to as "AF"). Also, during reproduction, the AT and the AF are necessary. There are various methods for such AT and AF techniques, but generally there is adopted a method whereby a differential between a plurality of light receiving surfaces of a photodetector for focusing and tracking which receive light reflected by or transmitted through a medium is taken to thereby obtain a focusing error signal and a tracking error signal, and an objective lens is driven by actuators for AT and AF.
FIG. 1 of the accompanying drawings shows the relation between a tracking error or a focusing error (the horizontal axis) and the reliability of the data of the then reproduced signal (the vertical axis). The time when the horizontal axis is 0 is the time when the light beam is on the target track or in the target focus state.
Now, during the recording and reproduction as described above, when there is a defect in the medium or when shock is applied from the outside, abnormality (i.e., the phenomenon that AT fails and the light beam does not correctly trace the information tracks) may arise in tracking servo. When such AT failure occurs, there has been the undesirable possibility that the objective lens moves randomly to thereby damage an optical head. Also, particularly when AT failure occurs during recording, there has been the problem that the light spot strays on the medium and superposed writing is effected on the recorded information with a result that the recorded information disappears.
Therefore, U.S. Pat. No. 4,669,072 proposes an optical information processing apparatus in which means for detecting the AT failure as described above is provided so that recording of information may De stopped when AT failure occurs. If such an apparatus is used, the problem attributable to the AT failure can be solved.
However, the trouble attributable to the abnormality of the servo has occurred not only in the aforedescribed AT, but also in AF. Particularly, if abnormality in focusing servo (i.e., the phenomenon that AF fails and the light beam becomes defocused on the medium) occurs during the recording of information, information which ought to be recorded is not actually recorded. If at this time, the information is not stored in a back-up memory or the like, this information will be lost permanently.